Conventionally, a drive unit such as a motor is used in an electrically controlled throttle apparatus to control a throttle valve to be in a predetermined throttle position in accordance with an accelerator position of an accelerator pedal stepped by a driver. According to an electrically controlled throttle apparatus disclosed in JP-A 10-047520, JP-A 2001-263098 and JP-A 2001-303983, a bore wall part and a motor housing part are integrally molded of a resinous material to construct a throttle body. Besides, according to JP-A 09-032590 and JP-A 11-132061, a throttle body has an outer pipe and an inner pipe that are coaxially arranged with each other to form an integrally molded double-pipe structure, in which the inner pipe receives a throttle valve.
The electrically controlled throttle apparatus drives a motor to control a throttle position of a throttle valve in accordance with an operating amount of a driver. In this throttle apparatus, circularity of an inner periphery of the substantially cylindrical bore wall part of the resinous throttle body has a large influences with an airtightness of the throttle valve, when the throttle valve is in its full close position. Besides, when the substantially cylindrical bore wall part, a substantially cylindrical valve bearing part and a substantially cylindrical motor housing part are integrally molded of a resinous material, contraction may arise in the valve bearing part and the motor housing part during its molding process. Accordingly, the inner periphery of the bore wall part is apt to be deformed due to the contraction of the valve bearing part and the motor housing part.
FIGS. 13 and 14 are perspective views showing an example of a throttle apparatus. The throttle apparatus has a throttle body 100 that is integrally molded of a resinous material with a substantially cylindrical valve bearing part 103, a substantially cylindrical motor housing part 104 and a bore wall part 105. The valve bearing part 103 rotatably supports one end side of the throttle shaft 102 that integrally rotates with a throttle valve 101. The motor housing part 104 receives a driving motor that rotates the throttle shaft 102 and the throttle valve 101 integrally. The bore wall part 105 internally forms an air intake passage, through which intake air flows into an internal combustion engine. The bore wall part 105 has a double pipe structure, in which a bore inner pipe 107 is coaxially arranged inside a bore outer pipe 106. The outer periphery of the bore inner pipe 107 and the inner periphery of the bore outer pipe 106 are connected with each other via an annular connecting part 109.
FIG. 15 is a partially cross-sectional perspective view showing the throttle apparatus. The bore wall part 105 having a double pipe structure is integrally molded of a resinous material. The annular connecting part 109 may has radial thickness that is less than its board thickness (axial board thickness) in its axial direction. The annular connecting part 109 has axial board thickness that may be greater than radial thickness of the bore inner pipe 106. In these cases, contraction of the bore outer pipe 106 in its molding process may cause deformation of the inner periphery of the bore inner pipe 107. Alternatively, the valve bearing part 103 and the sidewall of the bore outer pipe 106 may be directly connected with each other. In this connecting structure, contraction of a portion in the bore outer pipe 106 around the valve bearing part 103 may cause deformation of the inner periphery of the bore inner pipe 107. Alternatively, the sidewall of the motor housing part 104 and the sidewall of the bore outer pipe 106 may be directly connected with each other. In this connecting structure, contraction of a portion around the motor housing part 104 may cause deformation of the inner periphery of the bore inner pipe 107. Accordingly, when the motor housing part 104, the valve bearing part 103 and the bore wall part 105 are integrally molded of a resinous material, circularity of the inner periphery of the bore inner pipe 107 may be degraded due to contraction of the bore outer pipe 106 and the motor housing part 104. In this case, interference may arise between the throttle valve 101 and the bore inner pipe 107 when the throttle valve 101 rotates from its full close position to its full open position. Furthermore, a gap, which is formed between the inner periphery of the bore inner pipe 107 and the outer circumferential periphery of the throttle valve 101 when the throttle valve 101 is in its full close position, may become larger than a predetermined amount. Accordingly, airtightness may be degraded when the throttle valve 101 is in its full close position, and leakage of intake air increases in an idling operation of the engine.